Modern society has quickly adopted, and become reliant upon, electronic devices, such as, for example handheld devices for wireless communication. For example, cellular telephones continue to proliferate in the global marketplace due to technological improvements in both the communication quality and device functionality. These wireless communication devices have become common for both personal and business use, allowing users to transmit and receive voice, text and graphical data from a multitude of geographic locations. The communication networks utilized by these devices span different frequencies and cover different transmission distances, each having strengths desirable for various applications.
Cellular networks facilitate wireless communication over large geographic areas. These network technologies have commonly been divided by generations, starting in the late 1970s to early 1980s with first generation (1G) analog cellular telephones that provided baseline voice communication, to modern digital cellular telephones. Global System for Mobile Communications (GSM) is an example of a widely employed 2G digital cellular network communicating in the 900 MHz/1.8 GHz bands in Europe and at 850 MHz and 1.9 GHz in the United States. This network provides voice communication and also supports the transmission of textual data via the Short Messaging Service (SMS). SMS allows a wireless communications device (WCD) to transmit and receive text messages of up to 160 characters, while providing data transfer to packet networks, Integrated Services Digital Network (ISDN) and Plain Old Telephone Service (POTS) users at 9.6 Kbps. The Multimedia Messaging Service (MMS), an enhanced messaging system allowing for the transmission of sound, graphics and video files in addition to simple text, has also become available in certain devices. Soon, emerging technologies such as Digital Video Broadcasting for Handheld Devices (DVB-H) will make streaming digital video, and other similar content, available via direct transmission to a WCD. While long-range communication networks like GSM are a well-accepted means for transmitting and receiving data, due to cost, traffic and legislative concerns, these networks may not be appropriate for all data applications.
Short-range wireless networks provide communication solutions that avoid some of the problems seen in large cellular networks. Bluetooth™ is an example of a short-range wireless technology quickly gaining acceptance in the marketplace. A 1 Mbps Bluetooth™ radio may transmit and receive data at a rate of 720 Kbps within a range of 10 meters, and may transmit up to 100 meters with additional power boosting. Enhanced Data Rate (EDR) technology, which is also available, may enable maximum asymmetric data rates of 1448 Kbps for a 2 Mbps connection and 2178 Kbps for a 3 Mbps connection. In addition to Bluetooth™, other popular short-range wireless networks include for example IEEE 802.11x Wireless LAN, Wireless Universal Serial Bus (WUSB), Ultra Wideband (UWB), ZigBee (IEEE 802.15.4 and IEEE 802.15.4a), wherein each of these exemplary wireless mediums have features and advantages that make them appropriate for various applications.
As an example, IEEE 802.11a, 802.11g, and HiperLAN/2 wireless local area networks (WLANs) employ orthogonal frequency division multiplexing (OFDM) for the transmission of multiple sub-carriers in parallel to a receiving station or device (STA), enabling improved resistance to an interfering RF signal, since only those subcarriers having the same frequency as the interfering signal will be affected. OFDM divides the frequency spectrum into a number of equally spaced subcarriers, each of which carries a portion of a user's information. Each OFDM subcarrier is orthogonal with every other subcarrier, meaning the peak of one sub-carrier coincides with the null of an adjacent sub-carrier and they do not interfere with each other. User data may be modulated at the transmitting station or device onto the subcarriers by adjusting the subcarrier's phase, amplitude, or both. Either phase shift keying (PSK) or quadrature amplitude modulation (QAM) can be employed to modulate a binary one or zero onto the subcarrier. An OFDM system takes an input data stream and splits it into N parallel component streams, each at a rate 1/N of the original rate of the input data stream. Each component stream is then mapped onto a subcarrier at a unique frequency and combined together using the inverse fast Fourier transform (IFFT) to yield an OFDM symbol to be transmitted in the time-domain to a receiving station or device.
Carrier Sense Multiple Access/Collision Avoidance (CSMA-CA) is based on packet contention, and is the primary medium access method employed by IEEE 802.11 WLANs. It allows each transmitting station or device to contend for the shared channel by sensing whether the channel is in use, before attempting to transmit information to a receiving station or device. When a packet of information is to be sent, the transmitting device determines if the channel is clear, i.e., that no other device is transmitting at that moment. If the channel is clear, then the packet is sent. If the channel is not clear, the transmitting device waits for a random period of time and then determines again whether the channel is clear. If the channel is found to be clear on the second determination, the transmitting device transmits the packet. If the channel is not clear, then the process is repeated.
Each packet transmitted according to IEEE 802.11x WLAN protocols is typically in the form of a medium access control (MAC) frame consisting of a MAC header, a frame body, and a frame check sequence. The MAC header has thirty octets of overhead information including frame control, addressing for the transmitting and receiving devices, and sequence control information. The frame body following the header, contains the payload information, which may be management information, additional control information, or user data. The overhead represented by the MAC header is fixed for each packet, independent of the packet size. For large packets, the header represents only a few percent or less, but for small packets its size may be of the same order as the payload itself.
The contention based CSMA-CA channel access scheme is typically not efficient for aggregate system throughput in high-bandwidth systems, such as, for example Very High Throughput (VHT) WLAN systems. A large bandwidth is advantageous to support very high data-rate applications. However, there may be many devices in a Basic Service Set (BSS), each of which needs to contend for the shared channel to transmit relatively low-bandwidth, real-time traffic, such as, for example Voice Over Internet Protocol (VoIP) calls or real-time video. The frequent need to contend for channels to deliver high priority, time-critical data streams may cause significant inefficiency in spectrum usage, especially when short data packets must be transmitted with MAC headers as large as the frame body.
Real-time voice and video traffic are usually periodic in nature and a wireless Access Point (AP) has to typically deliver the streams within a short delay period to several non-AP wireless devices (STAs). Due to Quality of Service (QoS) constraints, real-time voice and video traffic can not be buffered for a long time, and hence the typical aggregation mechanisms in the IEEE 802.11n standard are not effective to reduce the number of channel contentions. When transmitting a Physical Layer Protocol Data Unit (PPDU) containing a payload of short packets (e.g., 200-1000 bytes), a large part of the PPDU time is taken by the PPDU preamble overhead (on the order of 20-48 us). Hence, frequent short packet transmissions can substantially reduce the effective throughput of the system.